1. Field of the Invention
The present invention relates to a power supply for a matal halide or like ac discharge lamp, and more particularly to such a supply for supplying substantially constant dc power to the lamp with alternating polarity.
2. Description of the Prior Art
Metal halide discharge lamps have severe power supply requirements. During lamp warm up after ignition, the lamp voltage is considerably lower than after the operating temperature has been reached, and the lamp current is considerably higher than during later operation. Furthermore, both during warm up and later operation, a maximum lamp power allowable can not be exceeded without damaging the lamp.
Thus both voltages and currents must be provided to the ac discharge lamp by an ac power supply so that the abovementioned requirements may be satisfied. Moreover, it is desirable to provide constant power to the lamp, so as to insure the constant light output and color temperature.
Conventionally, ballast-type power supplies have been used wherein an inductive ballast such as a choke and an inductor is connected in series with the lamp, and the voltage is controlled to drop in response to the amount of current drawn, so as to regulate the power output. In such ballast-type supplies, the inductive ballast itself is very bulky and massive. Further, in the conventional ballast-type supplies, a 200 volt source typically must be used for an ac metal halide lamp that is rated, for example, at 45 volts (higher than the related voltage). Thus to operate from a 100 volt ac line, a boosting transformer must be used in addition to the ballast inductor. Further since the amount of current to be stabilized by the inductor depends on the magnitude of supply voltage, taps must be provided to obtain constant voltage even when different line voltages are applied. As a consequence, all of these factors add to the bulkiness of such a ballast-type power supply and the complexity of the individual component part, and may lead to reduced efficiency.
The prior art, in trying to solve the problems associated with the ballast-type power supply, has employed a system which does not require an inductive ballast and yet effective for providing constant power to the ac metal halide lamp. An example of such power supplies is disclosed in Japanese Laid-Open Patent Publication No. 51-141488.
The power supply disclosed in the publication utilizes a switching regulator, as shown in FIG. 1.
In the power supply, the line voltage supplied between input terminals A and B is rectified through a dc power circuit 1, and the direct current thus obtained is periodically turned on and off by a switching transistor 2. Then the high frequency component of the current is removed by a low pass filter 6 consisting of a diode 3, an inductor 4 and a capacitor 5. Specifically, the outputs from each sensing circuit of a dc voltage sensing circuit 7 and a dc current sensing circuit 8 both provided in the output line of the low pass filter 6 are summed in a summing circuit 9. The summed output voltage is then inputted to an error detector circuit 10 which is provided in a control circuit for controlling the on-time of the switching transistor 2. The output voltage is further compared with the reference voltage from a reference voltage generating circuit 11 so as to obtain an error signal indicative of the voltage difference between the reference voltage and the summed output voltage.
The error signal is then supplied to a voltage-pulse width converter 13 for altering the pulse width of a constant frequency pulse signal from an ocillator 12 which operates with the converter 13. The output voltage of the converter 13 is effective to control the on or off time of the switching transistor 2 so as to maintain the error signal at constant level. As a consequence, a substantially constant dc power may be supplied to the load. The dc power is then fed to a commutator circuit 14. A pair of transistors 15 and 16 and a pair of transistors 17 and 18 of the commutator circuit 14 are periodically alternately controlled to be turned on through a commutator control circuit 19 so as to connect the regulated dc power to an ac discharge lamp 20.
It is to be noted that during the lamp 20 warm up, a large amount of current will flow because of the small impedance of the lamp 20. However, the voltage and current are sensed through the dc voltage and current sensing circuits 7 and 8, and the power being supplied to the lamp 20 is controlled so as not to exceed the predetermined level. This is accomplished by decreasing the pulse width of the constant frequency pulse signal from the voltage-pulse width converter 13 and reducing the on-time of the switching transistor 2 so as to maintain the error signal from the circuits 7 and 8 at a desired level. When the normal operating level is reached, the impedance of the lamp 20 increases. However, a substantially constant power may be supplied to the lamp 20 by increasing the pulse width of the constant frequency pulse signal from the converter 13 and prolonging the on-time of the switching transistor 2. Accordingly, since the power supply utilizes a switching transistor 2, it is considerably smaller and lighter than those using inductive ballasts, as well as being highly efficient.
It should be noted, however, that even in such a switching regulator type supply it is necessary to adjust the dc voltage and current sensing circuits 7 and 8 or the summing circuit 9 so as to provide constant power to the discharge lamp 20. The adjustment must be accomplished under normal operating condition with the lamp 20 connected to the power supply. This may cause an electrical shock inadvertently during the adjustment.
An object of the present invention is to provide a power supply which is compact, lightweight and highly efficient.
Another object is to provide a power supply which is safe to use where there is no possibility of causing electrical shocks.